Add night mode to default design skin

Add night mode to default design skin like: etherscan or beaconcha.in.

More info:

1. https://medium.com/js-dojo/how-to-enable-dark-mode-on-your-website-with-pure-css-32640335474
2. https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/